The present invention relates to a mirror for use in a microlithography projection exposure apparatus having a substrate and a reflective coating, to a projection lens for use in a microlithography projection exposure apparatus, to a microlithography projection exposure apparatus and also to a method for correcting a surface form of a mirror having a substrate and a reflective coating.
Microlithography projection exposure apparatuses serve for producing microstructured components by means of a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a photosensitive layer with the aid of a projection optical unit. The minimum feature size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The smaller the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Nowadays, imaging light having a wavelength of 193 nm or imaging light having a wavelength in the extreme ultraviolet (EUV) range, i.e. 5 nm-30 nm, is principally used. When imaging light having a wavelength of 193 nm is used, both refractive optical elements and reflective optical elements are employed within the microlithography projection exposure apparatus. By contrast, when imaging light having a wavelength in the range of 5 nm-30 nm is used, exclusively reflective optical elements (EUV mirrors) are used.
In order to enable a good imaging of the structure-bearing mask onto the photosensitive layer, it is necessary for the imaging aberrations or the wavefront aberrations of the projection optical unit to be reduced as far as possible. Therefore, it is necessary to ensure the surface form of, in particular, the mirrors used within the projection optical unit with a high precision.
For correcting the surface form of EUV mirrors, it is known to at least partially locally remove the topmost layer of the reflective coating. However, the layer removal required in this respect is large with respect to the wavefront aberration to be corrected. For correcting the surface form of the mirror, the surface form of the substrate can also be adapted suitably, before the reflective coating is applied, by being locally removed. In this case, however, it is not possible to correct those wavefront aberrations which only arise as a result of the application of the reflective coating or as a result of local changes in the reflective coating during operation.